The present invention relates to an improvement in signal processing in a pulse meter and a pulse photometer used for evaluation of functions of a respiratory system and a circulatory system.
In medical fields, an apparatus called a photoplethysmograph for measuring a pulse rate on the basis of a waveform of a pulse wave is used. An apparatus for measuring an oxygen saturation (SpO2), an apparatus for measuring the concentration of dyshemoglobin such as carboxyhemoglobin and methemoglobin, and an apparatus for measuring dye concentration injected in the blood, and the like are known as pulse photometers for providing information on a pulse rate and a blood substance concentration. An apparatus adapted to measure an oxygen saturation SpO2 is called a pulse oximeter and is used extensively in medical fields and sport fields.
The principle of the pulse photometer is to determine the concentration of an object material from a pulse wave data signal obtained by causing a living tissue to transmit or reflect light beams, which have a plurality of wavelengths respectively correspond to different absorbances of the object material, and by continuously measuring an intensity of transmitted or reflected light. In a case where noises are mixed in pulse wave data, calculation of a correct concentration or pulse rate cannot be achieved. Consequently, there is an anxious that an erroneous treatment may be performed.
Japanese Patent Publication No. 1-153139A discloses a pulse oximeter having a band pass filter to reduce such noises. Japanese Patent Publication No. 10-507118T discloses a method of correlating two signals for the same purpose.
However, these techniques utilize adaptive filters, the characteristics of each of which are changed according to the fundamental frequency of a pulse wave. Thus, a closed loop configuration may cause unstable oscillation. Also, these techniques have problems that oscillations due to a transient response of the filter occur, and that a response time of the filter becomes long. Thus, it is difficult to generate synchronous sounds by stably measuring a pulse rate in a real time manner.